They are sneezing in Stockholm, throats are itchy in India and Irish eyes are streaming. From Algeria to Iceland and Hong Kong to Aberdeen, record numbers of people are suffering the misery of hay fever - and it's getting worse. The weather here may be about to break, perhaps giving us some respite from the pollen-thick air, but everything from global warming to air pollution is conspiring to make hay fever the number one global irritant.

The figures are truly remarkable. The number of people rubbing their eyes in British doctor's surgeries has risen fivefold since the 50s, and about a quarter of people in this country are now believed to be hay-fever sufferers. And we are not alone: similar trends have been seen across Europe. Cases of hay fever doubled or even trebled in Sweden and Finland during the 70s and 80s, while Swiss surveys show about one in 10 are affected - up from less than one in 100 when a similar count was made in 1926. A massive 40% of Australians and Americans now say they suffer from hay fever or similar allergies, and previously unaffected areas such as West Africa are also starting to report increased cases. Some of this rise will be down to more people complaining, but experts are agreed that being more health aware cannot account for it all. There's a whole lot more sniffing going on.

"It varies from day to day but last week was very, very busy," says Mukesh Patel, who runs Columbia Pharmacy, a small chemist shop in east London. He reckons that about 60 people came into his shop last week looking for hay-fever remedies. "Things have been much busier than they were last year."

So what's going on? Are more of us getting hay fever - or has the western world simply got nothing better to complain about? Are we simply getting more whingy?

Two things are clear about hay fever: it is rarely caused by hay, and it doesn't give you a fever. The pollen grains thrown into the air from grass and trees get up people's noses and into their eyes and throats, and then provoke an allergic reaction. Short of staying at home with the doors and windows sealed, or taking drugs - with the sideeffects that may go with them - to suppress the symptoms, there's little that sufferers can do to avoid it.

"Each year since 1999 has been fairly severe," says Beverley Adams-Groom, a pollen forecaster (and hay-fever sufferer) who works at the national pollen research unit at University College, Worcester. It's too early to say whether this year will be the most severe yet, she says, but the signs could be pointing that way. Pollen levels in the Midlands, where the frontline citizens of Nottingham, Oxford and Stoke-on-Trent regularly face the worst pollen counts the English countryside has to offer, have registered as "high" or "very high" on 16 of the past 18 days. "Overall the actual counts haven't been as high this year but the pollen has been very persistent," Adams-Groom says.

Her unit prepares and sells forecasts of how bad the pollen is likely to be in any given place, using a secret formula she was unwilling to share with the Guardian. Broadly speaking, the forecast combines the number of pollen grains found trapped on sticky paper inside 28 monitoring stations around the country, with weather forecasts, windspeeds and a little inside information about the nature of the grains themselves. Pollen is capable of travelling hundreds of miles on the breeze and regularly rises and falls in the atmosphere as the temperature fluctuates during the day. Heat makes the air around the grains rise, hence the pollen count at ground level usually peaks during the cooler times of early morning and from about late afternoon.

Part of the answer to the rise in numbers of hay-fever sufferers could be down to grass and, surprise surprise, global warming. Up to 95% of hay-fever sufferers can blame the pollen from humble grass for their irritation. Grass usually releases the bulk of its pollen in June and July, which makes about now the peak time of year for hay-fever cases. But Adams-Groom has spotted what could be a worrying trend. "This year it started early," she says. In fact, this year the unit's pollen plotters registered their first "high" count on May 30 and "very high" on June 5 - a full week earlier than usual. Records show that global warming has raised springtime temperatures by 1C to 1.5 C over the past three decades, and some scientists say this is now fooling our flora into blooming - and so releasing pollen - earlier; which means a longer pollen season.

The weather also plays a major role, and this summer's combination of early rainfall followed by regular spells of unbroken sunshine and warm temperatures is perfect for growing grass. And less effort battling the elements means the grass can devote more time to producing pollen. "Much more pollen is produced on a hot sunny day with a bit of wind," says Adams-Groom. Australian researchers have estimated that under the right conditions a field of rye grass can produce as much as half a tonne of pollen per hectare over the summer growing season. All of which is bad news for the people living and working downwind.

Leaving aside issues such as the weather, there may be any number of genetic and environmental factors at play in the long-term recorded rises. Like other allergies such as asthma, whether a person will suffer from hay fever is partly down to their genes and partly down to their environment. It tends to run in families, but the condition can also seem to come and go throughout a sufferer's life. It is not the pollen grains themselves that cause problems, but specific proteins on and around their surface called allergens. The body considers these proteins foreign invaders and mobilises its immune defences to fight back - some people's fight more viciously than others, hence some people are more badly affected than others.

Anecdotal evidence may suggest that the perfect pollen conditions of the past two weeks have triggered the symptoms in many people for the "first time". But rather than "developing" hay fever, these individuals are likely to have a mild form of the condition that is only triggered when pollen levels are unusually high, says Peter Burney, an allergies expert at King's College London. "Very high pollen levels will irritate people who are a little bit allergic," he says.

Research has shown that increases in hay-fever cases, like asthma, tend to follow western-style lifestyles. "There are relatively small amounts reported among people in the Chinese mainland but when you get to Hong Kong there's quite a lot," Burney says. "It's down to lifestyle, but exactly which element of lifestyle is the $64,000 question."

Air pollution is commonly blamed - polluting particles irritate the nose and throat, and pollen allergens can piggyback on them - but Burney says there is not a strong relationship between it and hay fever. Instead, he, believes that the "hygiene hypothesis" is to blame. Less exposure to bacteria among children growing up in increasingly sterile environments could make the immune system redirect its efforts towards harmless targets such as pollen allergens, he suggests. Research has shown that children growing up among the dirt and germs of farm animals, particularly cows, are less likely to suffer allergies, including hay fever. And younger children are less likely to be affected than their older brother and sisters, who cough and sneeze their germs all over them.

This may or may not be true, and the irritating truth is that we may never know the full reasons behind the global rise in hay fever. In the meantime, sufferers are best advised to stay indoors, pray for rain and think of it as an excellent excuse not to mow the lawn.

Where there's mucus ...

Sales of prescription anti-allergy drugs used to treat hay fever have soared in recent years, particularly in the UK, where some studies show that 30% of young people are thought to be sufferers - double the figure 15 years ago.

Drug firms have focused on Australia, New Zealand and the UK, which are said to have the highest instances of hay fever. In the UK, a crowded range of over-the-counter drugs are available, with nasal decongestants eye drops and other products promising sufferers relatively cheap, temporary relief.

In more acute cases, prescription hay fever treatment in Britain has been dominated by nasal steroids, while doctors in mainland Europe prefer to place greater emphasis on anti- histamine treatments where possible. Zyrtec, a drug owned by the Belgian drugs and chemicals giant UCB, is the top-selling antihistamine pill worldwide, though a rival pill made by the US firm Schering Plough dominates the lucrative north American market.

By the end of last year, the UCB's familiar drug packs were contributing to company profits of almost €500m (£350m), which are continuing to grow.

The drug has recently come off patent in Europe, leading to price reductions in many countries of around 20% as copycat drug manufacturers have begun producing replica pills to rival Zyrtec.

Benadryl - produced by the makers of Viagra, US firm Pfizer - is the UK's biggest selling antihistamine brand and is available over the counter in doses that promise eight-hour relief.

The recent high pollen count in Britain has also provided a boost for high-street chemists, with Boots and Superdrug said by City retail analysts to be enjoying an unexpected bonanza as hay-fever sufferers queue up for an assortment of eye drops, such as Opticrom or Livostin; homeopathic remedies such as Nelson's Pollenna and New Era Combination H; and nasal sprays such as Beconase. Simon Bowers